One-electron reduction of sulphonium salts in aqueous solution: a pulse radiolysis study

Abstract

Absolute rate constants have been measured for the reduction of 1-alkylthiolanium (1a–c), 1-alkylthianium (2a–e), 1-(n-carboxyalkyl)thiolanium (3a,b) and 1–(n-carboxyalkyl)thianium (4a,b) salts, and ω-dimethylsulphonioalkanoic acids (5a,b), including biologically active S-methyl-methionine (6) and dimethyl-β-propiothetin (5b), by hydrated electrons in aqueous solutions using the pulse radiolysis method. This reaction was found to occur very rapidly (k in the range 10⁹–10¹⁰ dm³ mol^–1 s^–1), the individual rate constants being dependent on the electron inductive power of the substituents on the positive sulphur centre. The sulphuranyl radical R₃S˙, a possible reaction intermediate, could not be detected within the 0.1 µs time resolution of the applied technique. No reaction was observed between S-methylmethionine and CO₂˙^– or Me₂ĊOH under experimental conditions. The one-electron reduction potential of this sulphonium salt has been estimated to be < –1.8 V. C-centred radicals produced upon the reduction of the cyclic sulphonium salts 1-methyl- and 1-ethylthiolanium and 1-methylthianium salts by e^–_aq were characterized by using p-benzoquinone as a radical scavenger. It was found that ring opening is the predominant process rather than the cleavage of methyl or ethyl radicals.